The use of x-rays in the diagnosis of medical ailments and for other purposes is well known. Typically, an x-ray machine is located in a medical facility and is arranged to expose an object to be x-rayed, e.g., a portion of the human body such as a leg, hand or internal organ, to a predetermined amount, or dose, of radiation. The amount of radiation which passes through the object being x-rayed depends upon the characteristics of the object. Thus, the amount of x-ray radiation passing through the object depends on the interior shadow cast by the object and, hence, is representative of an x-ray image of the object. A cassette containing a photosensitive film is positioned opposite the x-ray source. A conventional cassette typically contains one or more phosphor screens which are excitable by x-rays to produce radiation (light) to which the film is sensitive and which is primarily responsible for the formation of a latent image in the photographic film.
The x-ray film typically uses silver halide technology which is expensive and which may be subject to further wet chemical processing steps. Additionally, there is an associated time lag between exposure and PG,3 developing, after which it may be determined that another exposure is necessary to adequately image the area being diagnosed. An inadequate image could be caused, for example, by overexposure or patient movement during exposure. Film systems can only be used once, require a light-controlled environment and have associated disposal and environmental concerns due to the need for developing solutions.
Alternatively, the cassette may contain photosensitive media which can be read externally by excitation of the latent image by a laser source or by detection of the latent image by induction of charge movement. The remainder of the x-ray machine operates conventionally. However, the x-ray film, involving wet or dry chemistry, is not needed,
Solid state radiation detectors convert the information in the x-ray radiation into electronic information which may be stored and transmitted. The electronic information may be read directly from an electronic display apparatus, typically a conventional monitor, may be printed to form a hard copy of the image or appropriately stored on magnetic or optical media, for example.
Solid state radiation detectors, e.g., x-ray detectors, divide the image into a plurality of image pixels. Individual radiation detectors convert the received radiation into an electronic signal. Collectively all of the individual radiation detectors provide a matrix of electronic signals, representative of the image. Typically, solid state detection information is represented digitally. Solid state radiation detectors are well known in the art. One example is the solid state radiation detector described in U.S. Pat. No. 5,105,087, Jagielinski, Large Solid State Sensor Assembly Formed From Smaller Sensors.
The medical practitioner using the x-ray machine usually prescribes a predetermined "dose" of radiation to the object being x-rayed. This amount of radiation is dependent upon, for example, the type and thickness of the object being x-rayed. A chest x-ray of a 300 pound man might require more radiation than an x-ray of child's finger. The amount of radiation is controlled in the x-ray machine by varying the intensity of the radiation output and by varying the length of time which the object is exposed to the radiation. The intensity of the radiation output is typically controlled by varying the voltage and/or the current supplied to the x-ray generator. The length of time is typically controlled simply by turning the x-ray generator on and off, or activating and deactivating the x-ray generator. It is desirable to minimize the x-ray dose due to the detrimental effects of x-rays on humans and animals.
However, there is considerable variation in radiation output among typical x-ray machines. Ardran, G. M. et al, "Constancy of Radiation Output During Diagnostic X-Ray Exposure", 51 British Journal of Radiology 862-874 (November 1978) describes some typical variations in radiation output. Also, Okkalides, "Abberations [sic] in X-Ray Output Waveforms of Radiological Generators, 15 Journal of Radiology 248-251 (1992) describes how variations in the rectification of power supply voltages affect x-ray tube output.
U.S. Pat. No. 4,101,775, Ensslin et al, X-Ray Tube Current Stabilizing Circuit, describes a technique designed to stabilize the x-ray tube with a circuit which causes the filament current of a x-ray tube to vary inversely to the tube current so as to maintain constant tube current regardless of moderate variations in line voltage. The circuit described in Ensslin et al, however, corrects for only one variable affecting x-ray tube output.
U.S. Pat. No. 5,144, 141, Rougeot et al, Photodetector Scintillator Radiation Imager, describes an array of photodetectors in which the intensity of light striking individual photomultipliers varies dependent on the distance of the photomultiplier from the point where the incident radiation interacted with a scintillator to produce the initial light burst. Each sub-group, consisting of four photodetectors, has a detect and hold circuit for amplifying and storing signals generated by the photodetectors. Multiplexing means samples the inputs stored.
Other methods of x-ray detection use a photoconductive plate, such as a thick film selenium x-ray plate, which is exposed to x-ray radiation in the conventional manner as with silver halide film. The x-ray plate, as a result of the exposure to x-ray radiation, contains a latent image of the object being imaged. The x-ray plate is then scanned with a focused laser beam to read the latent image stored in the plate. Such a method and system for accomplishing this method is disclosed in U.S. Pat. No. 4, 176,275, Korn et al. The exposure of the x-ray plate to the x-ray radiation creating the latent image is entirely conventional. The scanning of the x-ray plate to read out the latent image is a completely separate step and is fundamentally different from the x-ray radiation exposure step. There are variations in intensity of the laser beam used to read out the latent image in the x-ray plate for which compensation should be made. One mechanism to adjust for these laser beam variations is described in co-pending U.S. patent application Ser. No. 08/011,332, Vogelgesang et al, Method and Apparatus for Reducing the Effects of Laser Noise and for Improving Modulation Transfer Function in Scanning a Photoconductive Surface.
An x-ray generator generally has an initialization period following being turned on or following its activation. During this initialization period, the amount of radiation generated is variable and indeterminate. Because of this variation, it is conventional to wait until the x-ray generator has stabilized before beginning to count the duration of the predetermined exposure time. This initialization time, or period, is added to the predetermined exposure time and the total time of activation is represented by the initialization time plus the exposure time.
This known technique for determining the activation time for the x-ray generator is greatly variable. Individual x-ray generators, even those of the same make and model, vary significantly in the length of the initialization time period. Further, the length of the initialization period is a function of the length of time since the x-ray generator was last activated, the temperature of the environment, the age of the x-ray tube and other factors. Thus, the precise amount of radiation produced by the radiation generator and to which the object is exposed is greatly indeterminate.
In a conventional process for production of an x-ray image to be interpreted by a radiologist, the experience of the medical technician operating the x-ray machine and a trial and error approach are combined to produce an image which will sufficiently disclose the interior of the object being x-rayed. While it is the object of the technician to produce an x-ray image with the minimum time exposure of the patient to x-ray radiation, many variables make the task difficult.